Microphones such as MEMS microphones comprise a perforated backplate and a flexible membrane. The backplate and the membrane establish electrodes of a capacitor. Received sound signals induce oscillations of the membrane. Due to corresponding induced oscillations of the capacity, acoustic signals can be converted into electrical signals. In order to improve the signal quality of MEMS microphones, double backplate microphones or double membrane microphones can be created. In double backplate microphones, the membrane is arranged between two perforated backplates. In double membrane microphones, a perforated backplate is arranged between two flexible membranes. In each case, a microphone is obtained that comprises two capacitors and provides a differential output port. A differential port comprises two terminals where each terminal mainly provides the same absolute value of a voltage or a current but with opposite polarity. When signals propagate via differential signal ports or signal paths, common mode disturbances can easily be eliminated.
Although microphones with differential ports provide a better signal quality, their use, contrary to simpler microphones with a single capacitor as an acusto-electrical transducer, has not yet been commercialized, thus not much work has been devoted yet to methods to interface such microphones. Therefore, unsolved problems exist related to receiving and amplification of the signal from a differential microphone.
Microphones providing differential signals are known from U.S. Pat. No. 4,757,545, US 2008/0310655 A1 or US 2010/254544 A.
As a differential MEMS microphone electrically presents two capacitors, its electrodes have to be biased. Accompanying interface circuitry connected to the electrodes of the capacitor may provide a bias voltage for the capacitor. In previous works biasing is done by connecting a resistance element having a large resistance between the electrodes and ground.
Problems connected with receiving and amplifying a differential microphone signal such as differential microphone biasing, amplifier gain definition, influence of the parasitic capacitances at the interface nodes, differential microphone capacitances impedance conversion, obtaining low noise and low cut-off frequency of the amplifier response exist.
The signal quality depends on the quality of the acoustic capacitors' bias voltage. Further, the signal quality depends on the quality of the common mode output voltage. What is needed is a differential microphone with an improved bias voltage of the microphones acoustic capacitors and a better defined common mode output voltage and a method for driving such a microphone.